Low carbon-to-metal ratio and nucleation of hexagonal moiré superlattices in onion-like graphitic-domains within the walls of carbon nanotubes by the pyrolysis of ferrocene in viscous boundary layers

Abstract

Developing advanced methods for the stabilization of low dimensional moiré superlattices and van Hove singularities in collapsed chiral single-wall carbon nanotubes (SWCNTS) or in partially collapsed carbon nano-onions (CNOs) is an experimental challenge, which has recently attracted significant research interest. Herein, we report the reproducible identification of stacking-faults consisting of hexagonal moiré superlattices in unusual hybrid structures comprising multiwall carbon nanotubes (MWCNTs) interconnected by carbon nano-onion (CNO)-like graphitic domains. These nanostructures were fabricated by employing a chemical vapour deposition approach based on the sublimation and pyrolysis of ferrocene in a viscous boundary layer created between a rough substrate-surface and an Ar flow. The use of low vapour flow-rates in the presence of viscous boundary layers allows to access growth pathways based on a low local carbon-to-metal ratio (LCM). Interestingly, we demonstrate the nucleation of hexagonal moiré superlattices with periodicity D ∼ 0.5 nm within or in the proximity of the intermediate CNO-like graphitic domains. Such a structural modification was investigated by means of TEM/HRTEM and Fourier transform (FFT) analyses. Comparative investigations were performed on CNT-films obtained by the pyrolysis of ferrocene and dichlorobenzene mixtures under comparable low LCM, accessible using Cl-radicals at a low sublimation temperature. In the latter, we demonstrate the stabilization of larger hexagonal moiré super-periodicities D ∼ 1.0 nm, with the intermediate CNO-modes derived from a time-dependent variation in the concentration of Cl-radicals in the pyrolyzing system.